1. Field of the Invention
The present invention relates to a printing apparatus in which an image is formed by applying a printing agent onto a printing medium from printing means having a plurality of printing elements arranged. In particular, the present invention relates to a method of, and a configuration for, adjusting printing position displacement of the printing elements.
2. Description of the Related Art
A printing apparatus having functions of a printer, a copier, a facsimile machine and the like, or a printing apparatus used as an outputting device of a complex electric device or a work station including a computer or a word processor is configured so that an image (including characters and the like) is printed on a printing medium such as paper and a plastic sheet based on image information (including character information and the like). Depending on the printing method, such a printing apparatus can be classified into an inkjet method, a wire dot method, a thermal method, a laser beam method and the like. Among the above, a printing apparatus using the inkjet method (an inkjet printing apparatus), which carries out printing by discharging inks from printing means (a printing head) onto a printing medium, has a number of superior characteristics when compared with other printing methods in that higher resolution is easily achieved, high speed printing is possible in an excellently silent state, and the price and costs are low. Thus, the inkjet printing apparatus has become popular in a wide range from office use to personal use.
In general, in an inkjet printing apparatus, a printing head, which is configured by integrally arranging a plurality of printing elements including ink discharge ports and liquid paths for supplying inks to the ink discharge ports, is used. In addition, to cope with color images, inkjet printing apparatus include printing heads for a plurality of colors in many cases.
FIG. 1 is a perspective view for describing an inner mechanism of a general inkjet printing apparatus. In FIG. 1, reference numeral 101 denotes ink cartridges. Here, four ink cartridges respectively storing black ink, cyan ink, magenta ink and yellow ink are prepared. Reference numeral 102 denotes a printing head, which is capable of discharging ink supplied from the ink cartridges 101 for the respective colors in the −Z direction in FIG. 1.
Reference numeral 106 denotes a carriage, which is capable of moving and scanning in the X direction in FIG. 1 with the ink cartridges 101 for the four colors and the printing head 102 mounted thereon. During the time when a printing operation is not carried out, or when a recovering operation of the printing head 102 or the like is carried out, the carriage 106 is caused to wait at a home position (h) shown by a dotted line in FIG. 1.
Reference numeral 103 denotes a paper conveying roller, which conveys a printing medium P to the sub-scanning direction which is the Y direction by rotating with spurs 104 while supporting the printing medium P. Reference numeral 105 denotes a pair of paper feeding rollers, which feeds the printing medium P, and which plays a role of pressing the printing medium P in common with the paper conveying roller 103 and the spurs 104.
The carriage 106 is in the h position (home position) in FIG. 1 before starting printing, and moves and scans in the X direction when a printing start instruction is received. At the same time, the printing head 102 carries out discharge of inks according to printing signals. The discharged ink droplets are placed on the printing medium P. When finishing printing on the printing medium up to an end portion positioned on the opposite side of the home position, the carriage 106 returns to the original home position, and repeats printing in the X direction again. Otherwise, printing can be carried out by the carriage 106 moving and scanning in the −X direction without returning to the home position. When the printing scanning for one time has been completed, the printing medium P is conveyed by the pair of paper feeding rollers 105 and the paper conveying roller 103 by a predetermined amount in the Y direction. By intermittently repeating the printing scanning and the conveyance operation as described above, an image is sequentially formed on the printing medium P.
FIG. 2 is a schematic view for describing an arrangement state of the discharge ports for two colors which are observed in a case where the printing head 102 is viewed from the Z direction. In FIG. 2, reference numeral 201 denotes one discharge port of a discharge port row A for discharging black ink, and reference numeral 202 denotes one discharge port of a discharge port row B for discharging cyan ink. In FIG. 2, the discharge port rows A and B respectively have discharge ports of L=12, and each of the ink discharge ports is arranged at 1/600-inch intervals in the Y direction. Therefore, ink is discharged from each of the discharge ports while the printing head 102 is moving in the X direction, so that an image is formed with a printing density of 600 dpi (dot/inch) in the Y direction. In FIG. 2, n1 to n12 are reference numerals denoting arrangement positions of the respective discharge ports. The ink discharge port 201 is n12 in the discharge port row A, and the ink discharge port 202 is n1 in the discharge port row B.
In the present example, an amount of ink discharged from each of the discharge ports is set at approximately 2 pl per a droplet. In addition, a discharge frequency for stably discharging this amount of the ink droplet is set at 30 KHz, and a discharge speed is set at approximately 20 m/sec. In addition, a speed in the main scanning direction (X direction) of the carriage 106 on which the printing head 102 such as the above is mounted is approximately 25 inch/sec. With this, an image is formed with a printing density of approximately 1200 dpi in the main scanning direction.
Incidentally, in the printing head 102 such as the above having a general configuration, it has been heretofore known that displacement of dots is caused on a printing medium mainly for the following reasons. First, nozzle rows for a plurality of colors vary due to inaccuracy in manufacturing. Second, the printing head 102 is inaccurately installed to the carriage 106 when mounting the printing head 102 to the carriage 106. Third, a timing gap occurs in a case where main printing scanning is bi-directionally carried out. Then, to correct such displacement of dot placement, various printing position adjusting means and methods have been already proposed and implemented.
A printing position adjusting method applied to an inkjet printing apparatus will be described below. In general, in an inkjet printing apparatus, a printing position adjusting mode is included for adjusting dot placement prior to carrying out a normal printing operation.
FIG. 3 is a flowchart for describing each process carried out by an inkjet printing apparatus and a user at the time of performing the printing position adjustment.
First, when a printing position adjusting mode is designated, the printing apparatus prints predetermined check patterns on a printing medium in Step S4601.
FIG. 4 is a diagram showing one example of the check patterns outputted in Step S4601. Here, nine patterns printed in the following manner are shown. Timing of discharge from the discharge port row B is shifted from timing of discharge from the discharge port row A on a pixel-by-pixel basis from +4 pixels to −4 pixels in order to align the dot placement from the discharge port rows A and B arranged in the printing head 102. In the printing apparatus in this example, adjusting resolution for aligning printing positions of the discharge port rows A and B is set at one pixel out of 1200 dpi(dot/inch), that is, approximately 21 μm, and each pattern is printed with a resolution equal to the adjusting resolution.
FIG. 5 is a view showing that patterns obtained by shifting the two timing respectively by +2 to 0 pixels, out of the nine patterns shown in FIG. 4, on an enlarged scale. In FIG. 5, black circles and white circles are dots printed by the discharge port row A and the discharge port row B, respectively. Since the black circle dots printed by the discharge port row A are placed by discharging ink at the same timing, the black circle dots are printed in the same position in the main scanning direction. In contrast, since the white circle dots printed by the discharge port row B are placed by discharging ink at the timing of being shifted for one pixel, the while circle dots are also placed by being shifted on a pixel-by-pixel basis in the scanning direction in each of the printed patterns. In the present patterns, + direction shows a state where the printing head discharges ink at further delayed timing while the printing head is moving in the main scanning direction.
In the patterns shown in FIGS. 4 and 5, a state shown by +1 becomes a state where lines formed by the two discharging rows are overlapped most, which is recognized as a pattern close to a straight line. That is, it can be determined that the discharging timing of the discharge port rows A and B are in a most congruent state. In contrast, in a state shown by +2 or 0, distances d1 and d3 between the two lines are printed at approximately 21-μm intervals, though the lines are in the opposite directions.
Referring to FIG. 3 again, in the following Step S4602, the user selects a pattern, which is the closest to a straight line, of the nine patterns to enter the information from the printing apparatus, the host computer connected thereto, or the like. In the present example, as described above, it can be determined that the pattern of +1 is the closest to a straight line, and the user enters this information.
In Step S4603, the printing apparatus stores the information entered in Step S4602 in a memory (for example, a rewritable non-volatile memory such as an EEPROM) in the main body. By this, the printing position adjusting mode has been completed.
When printing is next carried out, the printing apparatus adjusts discharging timing of the discharge port rows A and B based on the information stored in the memory. By this, an image can be formed in a state where the printing positions of the two discharge port rows A and B are optimized.
The method of adjusting printing positions of the discharge port rows A and B has been described above. However, in a case where inks of a plurality of colors are discharged, or where a printing head having a plurality of discharge port rows for each color is used, printing position adjustment is needed for further more discharge port rows. In such a case, it can be addressed by having a configuration in which timing of each of the discharge port rows is adjusted by synchronizing with the timing of the discharge port row A as a reference and the respective adjustment value data are stored. In addition, even though a single discharge port row is used, in a case of bi-directional printing where discharge is carried out in the forward and backward movements of the carriage, a printing adjusting mode for adjusting the timing of discharge in the forward movement and the timing of discharge in the backward movement can also be achieved by a similar pattern and flowchart.
By the above-described conventional printing adjusting method, it has been possible to adjust printing positions between the plurality of discharge port rows and printing positions at the time of the bi-directional printing. However, it has not been possible to adjust printing positions in a single row. In recent inkjet printing apparatuses, demand for high-definition images comparable to film photos has increased, and further minimization of droplets and further enhancement of high-definition of the printing element arrangement have been in progress. Then, in such circumstances, situations have arisen where slight position displacement or a slight inclination of the discharge port row arranged in one row on the printing head cannot be neglected. In particular, an inclination of the discharge port row largely affects an image.
Under such circumstances, several methods of correcting adverse effects on an image due to an inclination of a printing head have been invented. In Japanese Patent Application Laid-open No. 7-309007, there has been disclosed an inkjet print system in which a displacement correcting circuit is provided to add offsets to image data to be printed by each discharge port in order to reduce displacement of printing positions caused by rotation of a printing head. In addition, in Japanese Patent Application Laid-open No. 7-40551, there has been disclosed an inkjet printing apparatus in which a plurality of discharge port rows arranged on a printing head is divided into a plurality of blocks so as to adjust the discharging order and intervals of each discharging block according to the inclination. Moreover, in Japanese Patent Application Laid-open No. 11-240143, there has been disclosed a method to correct displacement of printing positions in joint portions of each printing scanning caused by the inclination of the head. For that purpose, first, an offset amount is set from a displacement amount between a printing position by the discharge port on the uppermost portion and a printing position by the discharge port on the lowermost portion. After that, for one portion of the discharge ports, printing is carried out by shifting data by an amount based on the offset amount. Moreover, in Japanese Patent Application Laid-open No. 2004-9489, there has been disclosed an inkjet printing apparatus having means for changing allocation of data to be printed by each discharge port according to the inclination of the printing head.
However, in a conventional inkjet printing apparatus, though it has been possible to correct the inclination of a printing head or each of printing position displacement between respective discharge port rows, batch correction of complex printing position displacement caused by various causes has been difficult. For example, when the above-described printing position adjustment between respective rows is carried out, there has been a disadvantage that normal adjustment cannot be achieved because the user is confused or cannot select a proper value when it is in the state of including inclination in each discharge port row.
The above-described problem will be briefly described below.
FIG. 6 is a view showing a configuration similar to the configuration shown in FIG. 2 but having the printing head 102 with an inclination θ in each of two discharge port rows. In the present example, n1 of the discharge port row A is arranged in the position away from n12 by approximately 63 μm in the +X direction. This distance corresponds to a distance for approximately three pixels in the printing apparatus in which printing is carried out at 1200 dpi. On the other hand, n1 of the discharge port row B is arranged in the position away from n12 by approximately 63 μm in the −X direction. This distance also corresponds to a distance for approximately three pixels in the printing apparatus in which printing is carried out at 1200 dpi.
FIG. 7 is a diagram showing a printing state where check patterns similar to those in FIG. 4 are printed by using the printing head shown in FIG. 6.
FIG. 8 is a view showing that patterns of −1 to −3, out of the nine patterns shown in FIG. 7, on an enlarged scale. In FIG. 7 and FIG. 8, there is no case where lines by the two discharge port rows are recognized as a straight line like the pattern of +1 shown in FIGS. 4 and 5. The case where the distance between the two lines actually becomes the smallest is the pattern of −2. However, even in this state, displacement of d2=63 μm at maximum is caused between the discharge port rows A and B. Then, in the patterns of −1 and −3 in which timing is shifted by one pixel from the pattern of −2, displacement of approximately 84 μm is caused in the opposite directions.
In this manner, in a case of the discharge port row having the inclination shown in FIG. 6, a difference of each pattern is difficult to be determined, and it is difficult for the user to select the proper pattern of −2. In addition, even in a case where the proper value −2 is selected, the printing positions of the two discharge port rows are kept including the displacement of approximately 63 μm in an image to be outputted thereafter.
Such displacement of dot placement in the discharge port row is caused by inaccuracy at the time of manufacturing a printing head, inaccuracy at the time of mounting the printing head on the carriage, an inclination of the discharge port face against a flat surface of the printing medium, or the like. Therefore, a printing apparatus or a printing head is manufactured with consideration of avoiding generating such inaccuracy as much as possible at the time of manufacturing or mounting thereof. However, slight inaccuracy caused in spite of such an effort to suppress is not allowed for a demanded high-definition image recently. The problem due to the inclination of the discharge port row has become a major problem in recent inkjet printing apparatuses in which discharge of small droplets is achieved.